Docs about logging

- Relates #558

Author: jvalkeal

spring-shell-docs/src/main/asciidoc/using-shell-customization-logging.adoc

@@ -26,3 +26,30 @@ logging:
     name: shell.log
 ----
 ====
+
+If you need different log levels.
+
+====
+[source, yaml]
+----
+logging:
+  level:
+    org:
+      springframework:
+        shell: debug
+----
+====
+
+Passing contiguration properties as command line options is not supported but
+you can use any other ways supported by boot, for example.
+
+====
+[source, bash]
+----
+$ java -Dlogging.level.root=debug -jar demo.jar
+$ LOGGING_LEVEL_ROOT=debug java -jar demo.jar
+----
+====
+
+NOTE: In a GraalVM image settings are locked during compilation which means
+      you can't change log levels at runtime.